This invention relates to a device and method for evaluation of periodic velocity fluctuation (particularly, the phase angle and amplitude thereof) in a rotary drive such as a direct-current (DC) motor, a device and method ,for setting a corrective manipulated variable appropriate for the control of the periodic velocity fluctuation of the rotary drive based on a result of the evaluation, a control device and method for using the set corrective manipulated variable to control the drive of the rotary drive, and a program for executing the evaluation of the periodic velocity fluctuation of the rotary drive in a computer.
Heretofore, an ink jet printer has been provided with a carriage mounting a recording head thereon and capable of being moved to and fro along a guide shaft. Such a carriage is driven by a DC motor.
In a DC motor, even if the driving current or driving voltage is constant, the torque during one rotation of a motor shaft is not uniform because of structural reasons of the DC motor. Periodic torque fluctuation, so-called cogging, occurs. Accordingly, the rotational velocity of the DC motor is periodically fluctuated. As a result, the moving velocity of the carriage being moved to and fro by the rotation of the DC motor is fluctuated as well.
As the moving velocity goes up, the ink ejected from the recording head during such a movement appears to be thin (light-colored), since the ink is ejected with a relatively lower density of placement on a recording medium. To the contrary, when the moving velocity of the carriage goes down, the ink ejected from the recording head appears to be thick (dark-colored), since the ink is ejected with a relatively higher density of placement on a recording medium.
In other words, when the moving velocity of the carriage is fluctuated along with the fluctuation in the rotational velocity of the DC motor, a light-colored area and a dark-colored area alternately appear and produce a striped pattern on the recording medium, even though the overall area should be recorded with a relatively constant density.
In order to avoid the above problem, periodic signals having the same period as the period of cogging, but having the opposite phase, are superimposed on the drive signal of the DC motor. This generates a torque having an opposite phase to the cogging torque in the motor and cancels the periodic velocity fluctuation of the DC motor.
Since it is necessary to generate a signal having a phase opposite to the rotational velocity fluctuation to be canceled for the cancellation of the periodic velocity fluctuation of the DC motor, the following technique has been proposed for example. That is, firstly, in synchronization with the rotation of the DC motor, a predetermined number of pulses are generated in one rotation of the DC motor. The period of each pulse is measured to be compared with a reference period when the motor is rotated at a predetermined rotational velocity, so as to obtain rotational velocity error data per pulse period. Further, each rotational velocity error data is averaged with rotational velocity error data for the previous rotation so as to obtain average rotational velocity error data per pulse period. Based on the pulse having the maximum error data from among the respective average rotational velocity error data, the phase of the signal (sine wave) to be added to the driving signal is set.
However, in the above proposed technique, a plurality of average rotational velocity error data are obtained per one rotation of the DC motor. Then, from the obtained error data, the phase of the rotational velocity fluctuation of the DC motor is specified to set the phase of a signal for rotational fluctuation cancellation to be added to the driving signal. Therefore, it is possible to set a sine-wave phase for controlling the velocity fluctuation in the case in which one rotation of the DC motor corresponds to one period. However, for example, it is not possible to specify the phase of the periodic velocity fluctuation generated by the load connected to the motor shaft of the DC motor, such as a belt or pulley connected to the motor shaft to move the carriage (i.e., periodic velocity fluctuation generated by the rotation of the belt or pulley). That is, it is not possible to control a periodic velocity fluctuation that does not synchronize with a rotation of the DC motor.